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A leaf scar is the mark left by a leaf after it falls off the twig. It marks the site where the petiole attached to the stem. A leaf scar is typically found below a branch, as branches come from axillary buds located above leaf scars. [citation needed] The leaf scar on Ailanthus altissima
Below the leaf scar the leaf-cushion tapered to a basal position. In this tapering area, circular impressions with fine pits were present. These impressions were continuous with the parichnos scars near the top of the tapering portion. This is because the impressions are formed by aerenchyma tissue that developed in closely with the parichnos.
The actual leaf scar is present slightly above the midpoint of the cushion and is roughly elliptical in shape. On the leaf scar, three small pitted impressions can sometimes be found. The central and always present pit results from a vascular bundle that extended into the leaf from the stem, known as "parichnos," a system of aerating tissues.
Conidia are then produced inside the pycnidia and can be released when conditions are favorable: i.e. humid weather. As a result, vulnerable tissues are at risk of initial invasion throughout most parts of the year, although it most commonly occurs in autumn and spring following winter injury and leaf scars.
The C-shaped leaf scars of white ash are useful in distinguishing this species from the closely related green ash (Fraxinus pennsylvanica). The lateral buds of F. pennsylvanica share a less curved margin with their corresponding leaf scars, imparting an overall D shape to the latter. [8]
The leaves are opposite and palmately compound, with 5–7 leaflets 13–30 cm (5–12 in) long, making the whole leaf up to 60 cm (24 in) across, with a 7–20 cm (3–8 in) petiole. The leaf scars left on twigs after the leaves have fallen have a distinctive horseshoe shape, complete with seven "nails".
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At the end of each branch is a spiral cluster of long, linear leaves with a pectinate (comb-like) edge tapering to a long point at the apex. This margin is filled with small reddish colored teeth. The leaves are simple without lobes and can be up to 2 metres (6.6 ft) long and 3 to 11 centimetres (1.2 to 4.3 in) broad. They are without petioles ...